LTH
The Scintillator Project
Projektering – KET050
25/5-2012
Mikael Sjölin
Eric Skopal
Yousef Zaghi Valencia
Andreas Åberg
Björn Öhrstrand
Tutors
Reine Wallenberg
Maria Messing
Staffan Hansen
Industry advisor/representative
Herfried Wieczorek
Abstract
A PET scan is used for full body imaging and the detection material is today made of scintillating
LYSO-crystals. The LYSO-crystals has grown too expensive, therefore a new material needs to be
found. Different materials such as ionic liquids, semiconductors, other inorganic crystals, etc. were
investigated. The investigation was done by reviewing articles and then based on the properties of
the different materials the search was narrowed down to a few possible candidates. After some
research Liquid xenon was considered to be the most promising material however with some special
technical applications. The liquid xenon needs a high purity level, therefore a purity system was
investigated. Also a cooling system (or pressurized) is required for the xenon to maintain in liquid
phase at room temperature. A cost and availability research was also performed. Liquid xenon is not
the ideal material because of its limits in cooling and purification requirements, therefore a deeper
investigation in semiconducting materials appears to be the future for PET- scanners. Possible
candidates to substitute liquid xenon can be semiconductors, for example ITO, CZT, etc, however not
as a traditional scintillator but instead together with techniques that are capable of using Compton
scattering information.
2
Contents
Abstract ................................................................................................................................................... 2
1.
Introduction ..................................................................................................................................... 4
1.1.
2.
3.
4.
Philips .................................................................................................................................. 4
Scope ............................................................................................................................................... 5
2.1.
The Problem ............................................................................................................................ 5
2.2.
Constraints .............................................................................................................................. 5
2.3.
Description of technical terms ................................................................................................ 6
2.3.1.
Scintillation ...................................................................................................................... 6
2.3.2.
PET-detector .................................................................................................................... 6
2.3.3.
LYSO:Ce ............................................................................................................................ 6
2.3.4.
DOI – Depth of interaction .............................................................................................. 7
2.3.5.
Compton scatter .............................................................................................................. 7
2.3.6.
Energy Resolution ............................................................................................................ 8
2.3.7.
Decay time ....................................................................................................................... 8
2.3.8.
Light yield ........................................................................................................................ 8
Method ............................................................................................................................................ 9
3.1.
Work procedure ...................................................................................................................... 9
3.2.
Work progress ......................................................................................................................... 9
Results and discussion ................................................................................................................... 11
4.1.
Liquid Xenon .......................................................................................................................... 11
4.1.1.
Liquid Xenon, Properties ............................................................................................... 11
4.1.2.
Technical implementation ............................................................................................. 12
4.1.3.
Cost and Availability ...................................................................................................... 16
4.1.4.
Challenges...................................................................................................................... 17
4.2.
Alternatives to liquid Xenon .................................................................................................. 18
4.2.1.
Possible alternatives ...................................................................................................... 18
5.
Conclusion ..................................................................................................................................... 20
6.
Reference ...................................................................................................................................... 21
3
1. Introduction
1.1.
Philips
Royal Philips Electronics (commonly known as Philips) is one of the worlds leading companies in the
production of electronics [1]. The company was founded in Eindhoven in 1891, by Gerald and Anton
Philips, where the first product was manufacturing of carbon filament lamps [2]. Today Philips is a
largely spread company with over 120 000 employees in over 100 different countries. It is
headquartered in Amsterdam, got a sales number of € 22.3 billion (2010) and is today listed on both
the AEX and NYSE stock exchange. The company is divided into 3 main divisions; Philips Consumer
Lifestyle, Philips Healthcare and Philips Lighting. In Philips Healthcare the development in nuclear
medicine has expanded a lot during the last decade [1]. Today Philips produces MR, SPECT/CT and
PET. Especially the PET technology has developed a lot during the last years [3]. In figure 1 a Philips
CT/PET-scan is illustrated
Figure 1. Philips current CT/PET-scan.
4
2. Scope
2.1.
The Problem
Today in PET-scanners, the scintillating material used is (Lu,Y)2SiO5:Ce (LYSO). As can be seen, this
material is heavily based on the rare-earth metals lutetium, yttrium and cerium. The problem with
LYSO crystals is that the price of rare-earth materials has grown too expensive. This is not only a
problem for yttrium and lutetium, but generally for all rare-earth compounds. Therefore a substitute
material is needed which is not based on rare earth materials. The production of rare-earth materials
is mainly positioned in China which more or less has monopoly on the market. There are other
possible mining locations in the world, for example in USA and Sweden. These mines are however
currently not in operation and it will take up to five years to reinitiate the mining after a decision has
been made to do so. Due to these facts the price of rare-earth metals has increased a lot during the
last years, and will probably continue doing so. To assure supply of base materials for PET
construction a new material is needed. [15]
The main goal for this project is to develop a material that has properties at least on par with LYSO,
but is not rare-earth based. The range of materials that scintillate is quite large, for example Ionic
liquids, heavy glasses, organic compounds, semiconductors and noble gases all show scintillating
properties. The desired solution could be to develop a new kind of inorganic crystal that is not rareearth based. Since these have been heavily used in a historical perspective the idea should not be
completely neglected. Another approach to solve the problem could be to leave the old idea which is
based on inorganic crystals and use a different kind of scintillating material. A problem with this
solution is that it probably would require a new kind of apparatus design. [16]
2.2.
Constraints
The new substitute material should have the following properties:













Density of 6.0 g/cm3 or higher.
Light yield of 30 000 photons/MeV or higher.
Decay time of 40 ns or shorter.
Should not be of rare-earth based material, thus has to be abundant and widely available.
Should be easily producible as a single crystal or as transparent ceramic material.
Emission wavelength between 400 nm to 1000 nm.
Should be stable in air for at least 60 minutes at a temperature range of 0 to 50°C.
Should have minimal afterglow.
Should be able to scale up for a production of 5 tons per year.
Should not have any side-effects endangering humans.
Should have a stopping power which stops at least 90% of the gamma rays.
Should have a cost less than 5$ per scintillating unit.
Should easily be implemented in Philips existing apparatus, without making any major
changes. [16]
The issue is that there are not any known ideal materials that show all these properties. Instead of
viewing these properties as definite requirements, the quest should be to use them as guidelines and
try to find a material that fulfills most of the properties stated above. [15]
5
2.3.
Description of technical terms
2.3.1. Scintillation
Materials that emit luminescence when struck by gamma-radiation are called scintillators. The
absorbed radiation excites electrons to a higher energy state, when the electrons de-excite back light
is emitted in the form of phosphorescence or fluorescence. Materials that scintillate are common in
lots of different forms, for example liquids, organic/inorganic materials and polymers. [14]
2.3.2. PET-detector
Positron Emission Tomography (PET) - detector is commonly used in the medical industry due to its
applications within medical imaging. The major application for PET-scan is to detect tumors. The
patient is injected with a radioactive substance. The task of the substance is to visualize biochemical
events by emitting positrons through decay [5]. The radioactive substance is usually glucose doped
with different radioactive isotopes. Oxygen-isotope, nitrogen-isotope and carbon-isotope can all be
used but has a disadvantage that the half-life time is too short [4]. However the fluorine18-isotope is
more desirable because the half-life is longer, approximately two hours [4]. Furthermore the
fluorine-isotopes are easily attached to the glucose.
It takes approximately 60 minutes after the injection for the substance to circulate throughout the
body [6]. Thereafter the patient is ready for the PET-scan which takes about 15-35 minutes
depending on the type of scan that is applied and the type of scanner being used [6]. When the
radioactive substance starts to decay, the emitted positron annihilates with an electron after
traveling a short distance within the subject [5]. When an electron collides with a positron,
annihilation occurs. This results in emission of two γ-photons that are sent in opposite direction of
each other and carries an energy of 511 keV each [5].
The gamma-photons that are emitted are re orded by dete tors la ed 3 0 around the atient.
2.3.3. LYSO:Ce
Currently the scintillators used in PET at Philips is LYSO:Ce [9]. These scintillators are used frequently
today because they exhibit desirable qualities for gamma-rays detection. The density is about 7.2
g/cm3 and the light yield is about 30 000 Ph/MeV [9]. The decay time is 40 ns which makes LYSO:Ce
good choice for PET. Unfortunately they are heavily based on rare-earth materials which price has
rapid increased in recent years and thus very expensive. [9]
6
2.3.4. DOI – Depth of interaction
The depth-of-interaction problem in PET instrumentation, also known as the parallax problem,
impacts the spatial resolution. The interaction of 511 keV photons with long crystal elements at
oblique angles and also the penetration of 511 keV photons through multiple crystal elements, as
shown in figure 2, lead to irregular spatial resolution. [5]
Figure 2. Illustration of DOI effect on the spatial resolution[5].
2.3.5. Compton scatter
When a Com ton s attering o urs, the γ-ray loses a part of its energy to an electron through
scattering. This can be seen in figure 3. The lost energy is absorbed in the crystal. [8]
When detecting the photons there is usually an energy window set around the 511 keV photo peak.
Therefore if a photon has lost some energy on its way to the detector it will not be measured. If the
user wants to collect more photons the energy window has to be broadened. The increased energy
window will results in a worse energy resolution which is not desirable. Compton scatter is therefore
not wanted in scintillators [5].
Figure 3. Compton scattering[10].
7
2.3.6. Energy Resolution
It is important to be able to distinguish the photons with the energy of 511 keV from the other
photons that has lost energy through Compton scattering. In order to do so the Energy Resolution
has to be at a high level which is accomplished when the energy window is narrowed. The Energy
Resolution is defined as the full width at half of maximum (fwhm) at a certain energy as seen in
figure 4. [11]. In figure4 the x-axis represent the energy content in the photons and the y-axis is the
relative share of each energy [13].
Figure 4. Definition of energy resolution [13].
2.3.7. Decay time
The luminescence decay after material excitation by a high energy photon is called the scintillation
decay. The decay curve I(t) is defined in the following formula:
( )
Were
[12]
∑
[
⁄ ]
is the Decay time which defines the speed-of-light-response when studying the material.
2.3.8. Light yield
When a scintillation material is hit by a high energy photon or particle electrons will be excited.
When they relax luminescence occur. The light yield is defined as the number of photons emitted per
1 MeV applied to the material. [12]
8
3. Method
3.1.
Work procedure
At the start of the project the search was open minded to what kinds of materials that could be used,
some examples are listed below






Semiconductors both as a scintillating material and photoelectric detector
Ceramics with doping of scintillating substances
Liquids, for example ionic liquids and xenon
Organic compounds (polymers)
Heavy glasses
Other crystals (for example SrI2,PbI2)
Information about the different properties of the materials was gathered by reviewing articles. The
most promising and interesting candidates have been studied more in detail.
3.2.
Work progress
To investigate whether a material was suitable for PET-applications articles were reviewed. When
some properties were not good enough for the application the material was discarded. Some of the
key properties of the materials that were studied are listed in table 1.
Table 1, Properties of some materials [17] [18] [19] [20] [21] [22] [23]
Material
Semiconductors
Heavy glasses
Ionic liquids
Strontium iodide
Liquid Xenon
Light yield
(ph/MeV)
Very low
800
68000-120000,
(21000 ceramic)
40000
Density (g/cm3)
Wavelength (nm)
Decay time
6
4-6
4.5
400
420
1 ns
100-1000 ns
μs – ms range
1.7 μs
3
178
45
When semiconductors were studied just a few was studied of the many that exist. Some examples of
semiconducting materials that have been studied are zinc oxide doped with gallium, cadmium sulfide
doped with indium and lead iodide. For a closer look at the properties of semiconductors, see table
1. The properties that make semiconductors a promising alternative is that the decay time is around
1ns, the density is around 6 g/cm3 and that the emission wavelength is around 400nm. The
disadvantage is however the light yield. The light yield decreases drastically with increasing
temperature, this is due to nonradiative recombination that occur at high temperature. It makes the
light yield very low at room temperature and rule out semiconductors as traditional scintillators, at
least until a mechanism is found to counter the decrease of light yield with increasing temperature.
[17] [18]
The heavy glasses that have been studied include for example silicon glasses, boron oxides and flint
glasses doped with a scintillating material like europium, terbium and cerium. The glass material
itself is just a carrier material to the doped material to increase the density. Depending on what type
of glass and doped material that is used the density vary between 4 to 6 g/cm3. The downside is the
9
decay time that lies around 100-1000 ns and the light yield is around 800 ph/MeV, which is very low.
The decay time and light yield is the main reason that heavy glasses are not further investigated. [19]
[20]
The scintillating ionic liquids that have been studied consist of anions and cations. Where the anion is
usually bromine, chlorine, manganese etc and the cation consist of long chain of organic molecules.
The main problem with ionic liquids is that the decay time is very long. The decay time varies from
microseconds all the way up to milliseconds. Because of this the ionic liquids is not a good candidate
to use as a scintillator for PET. No information about the light yield has been found and experiments
must be made to gather more information. [21]
A material that has some very promising properties at first glance is strontium iodide. The light yield
is as high as 100 000 ph/MeV depending on if it is in crystal form or ceramic form, with the highest
values in crystal form. Other good properties are the emission wavelength and the density. A
problem is that the decay time is around one microsecond and that the strontium iodide is
hygroscopic. Because of the decay time and its hygroscopic properties strontium iodide is not a good
scintillating material for PET at this time. [22] [23]
10
4. Results and discussion
4.1.
Liquid Xenon
Liquid xenon has proved itself a very interesting material working as scintillating element in PET
scans. Liquid xenon was a known scintillator already in the early 1970s [24]. The research about
liquid xenon as a scintillator was reborn in the 2000s, this due to the fact that the price of scintillating
material used in PET scans have increased a lot. The properties of liquid xenon are indeed interesting
in PET scan applications. The light yield is comparable with the LYSO crystals and the decay time is
even better than the crystals used today. The better decay time results in better energy resolution
which is desirable. Still there are some problems when using liquid xenon as a scintillator, the usage
of liquid xenon requires cooling because the boiling point of xenon is -1080C (at 1 atm). The cost of a
full scale human PET will be a large amount of money. Xenon is extracted from air, where it has a
concentration of 0,087±0,001ppm [25]. It is obvious that this separation will be expensive due to the
low concentration in air. Another challenge with using liquid xenon is the need of high purity, if the
purity is not high enough the light will be scattered and the light yield drops drastically.
4.1.1. Liquid Xenon, Properties
The main reason why Liquid Xenon is an alternative for usage as a detector material in PET is because
it has very promising physical properties for PET applications. If only considering the scintillating
properties, liquid xenon show a light yield of 46 000 Photons/MeV at -100 oC and 29 000
Photons/MeV at 1 oC[26].
The scintillating mechanism have different decay times, however the longest mechanism have a
decay time of ~45 ns and the shortest decay time is 2.2 ns.[27]
An important property for gamma ray detectors are their density. The density is normally important
because it decides how thick your scintillator must be to catch enough radiation to create a picture,
however, as will be seen below, if information is used properly, this is not necessarily a problem. [30]
The emission wavelength is 178 nm, which is a lot lower than the scintillation light from LYSO. There
are a couple of ways to solve this challenge, which will be presented later. [28][29]
Because of the characteristics of liquid xenon there are two ways of using liquid xenon as a gamma
ray detector, using only the scintillation light or use both the scintillating light and an ionization event
that takes place during absorption of energy. When liquid xenon is hit by a gamma ray, the energy
transferred from the photon excites electrons to higher energy levels. Some electrons are excited
enough to create an electron cloud that if left alone will relax and give scintillation light. If an electric
field is applied this cloud of electrons can be used to get extra information. If the electrons are
gathered instead of letting them relax, a part of the scintillation light will disappear. The part that is
lost is the light with a decay time of 45 ns, which accounts for approximately 70 % of the total light
output. This means that only 30 % of the light will appear if an electric field is applied, which is below
the required limit. Due to the design of the chamber, which will be explained later, this is not a
problem. Since the light removed is the light with a decay time of 45 ns (the longest), the decay time
is also lowered to the second longest event, which is 27 ns. [28][29]
11
4.1.1.1.
Advantages of liquid Xenon over LYSO
Because of the properties explained above, it is possible so get an energy resolution that is a lot
better than when using ordinary LYSO. The main reason for this is that a lot of extra information can
be extracted from the Compton scattering taking place. Normally the Compton scattering is an
unwanted process that only increases noise, which leads to blurry pictures. The idea with using liquid
Xenon is that it no longer is the scintillation light that is the main source of information, but instead
the cloud of electrons that is formed. [5]
When the initial gamma ray “hits” the liquid Xenon, an ele tron loud is formed together with
scintillation light. The light is then used to time the travel time of the electron cloud. Depending on
the size of the electron cloud, the energy deposited can be calculated. Because of this, it does not
matter if the interaction was Compton scattering or photoelectric effect.
As explained before a problem with normal scintillators is that a density that is high enough to stop
the gamma rays within a reasonable distance is needed. This is not necessarily a problem with liquid
Xenon because of the usage of Compton scatters. If at least three scattering points are measured the
initial scattering angle and energy of the incoming gamma ray can be determined by Compton
kinematics. This means that as long as the thickness of the Xenon allows for three scatters, all
necessary information can be gathered. Another advantage with mapping the Compton interactions
is that the problem with depth of interaction is eliminated.
4.1.2. Technical implementation
4.1.2.1.
Time projection chamber
The usage of liquid Xenon as a gamma ray detector in PET-scanners will require that the currently
used apparatus is modified. This is partly because the machine will not only register scintillation light,
but also an ionization current. Another big reason to why the apparatus needs to be changed is of
course that the machine will no longer use solid inorganic crystals, but instead a liquid that needs to
be kept at a very low temperature or high pressure.
The idea that seems most promising is to use a Time Projection Chamber, or TPC. The main idea is to
have a couple of larger segments filled with liquid xenon. Inside these segments there are anode
wires and induction wires, together with a light detector that detects the scintillation light. When the
gamma ray hits the liquid xenon, either a Compton scatter or energy deposit by photoelectric effect
occurs. This leads to the creation of an electron cloud and scintillation light as described in section
4.1.1. Because of electrical potential made of an applied electric field of approximately 2kV/cm the
electron cloud starts to travel towards the closest anode wire. The size of the current that hits the
anode wire makes it possible to determine how much energy that was deposited by the gamma ray.
The anode wire that was hit by the electron cloud also makes it possible to determine one coordinate
in space where the interaction took place. [28][27][34]
The light that was produced during the interaction is measured by the detector. If the time it took for
the light to reach the detector is very short it is possible to time the travels of the electron cloud to
the anode wire with the scintillation light. Since the fastest decay time of liquid xenon is 2.2 ns, which
is very fast compared to the speed of the electron cloud, this can be considered to be true. When the
light hits the detector a clock starts and then stops when the electron cloud hits the anode. Since the
12
speed of the electron cloud is approximately 2.2 µm/s the traveled distance can be determined. This
gives another coordinate in space to where the interaction took place. [28][27]
The third and last coordinate is determined when the electron cloud travels past the induction wires.
When this happens a current is created in the relevant wire, and thus the last coordinate is given.
[28][27]
The idea with a couple of larger segments filled with liquid xenon can be seen below in figure 5. An
overview of the wire setup can be seen in figure 6.
Figure 5. Sketch of a PET with larger segments that will be filled with liquid xenon. [32]
Figure 6. The placing of wires and strips inside the segments filled with liquid xenon.[27]
The Frisch grid, which is made of a conductive material and is on ground potential, is placed between
the anode and cathode to increase the time resolution. If the Frisch grid is placed correctly, the
electric pulse can be measured when the electron cloud passes the Frisch grid.[33]
13
A problem with using liquid xenon in the way described above is that it has to be cooled to a
temperature of -108 oC if it is at atmospheric pressure. It is very hard to get a homogenous cooling if
the tubes are not inserted in the liquid xenon, and if the cooling tubes are placed inside the liquid
xenon it would probably result in faulty data from the electron cloud. A non homogenous cooling
would result in a large decrease in attenuation length for the scintillation light and electron cloud
which would give a big loss in data collecting and thus energy resolution and overall picture quality.
An alternative to cooling would be to pressurize the xenon to make it stay in liquid form even close
to room temperature. However, this would result in another problem; the density at normal pressure
and -108 oC is 3.0 g/cm3. If the xenon is pressurized to 56 bar it would be possible to have it in liquid
form close to room temperature, but with the decrease in density to ~1.9 g/cm3. This would mean
that a thicker layer of liquid xenon is required to stop enough gamma rays. The probability of
achieving at least one Compton scatter for a 1.157 MeV gamma ray is 79 % if the thickness of the
xenon is 12 cm at 1 atm. With a density of 1.9 g/cm3 it would be hard to create practical machine.
[26][35][36]
The scintillation light of liquid xenon has a wavelength of 178 nm. Since this is outside of the range
that normal light detectors work within either an uncommon detector that probably is more
expensive must be used, or a wave shifter can be placed in the liquid xenon to change the
wavelength of the light. A light detector that is able to detect light at 178 nm is the Avalanche photo
diode, or APD. This detector has been used successfully in experiments and has shown a quantum
efficiency of ~100 % for xenon scintillating light (178 nm). [37][39]
The other alternative is to use a wave shifter. For example p-terphenyl has shown promising results
of shifting liquid xenon scintillation light from 178 nm to around 400 nm with a quantum efficiency of
100 %. A way to implement the p-terphenyl would be to for example place in on the inside of the
xenon containers. When the scintillation occurs, the 178 nm light would be absorbed by the pterphenyl and then emitted again at the new wavelength. The decay time of p-terphenyl is very low
and will not increase the time it takes for the scintillation light to reach the detectors by a significant
amount. A problem with this solution could be that the p-terphenyl contaminates the liquid xenon
over time, and thus the performance of the PET-scanner. [38]
4.1.2.2.
Xenon purification system
Xenon is produced like most gases from distillation of air. First the O2 is removed from the N2, and
then further enrichment of a Krypton/Xenon mixture (either by adsorption or further distillation).
After this procedure, the last step is to separate the Krypton from the Xenon. [40]
For this application, very high purity level of Xenon is needed in order to get a good energy resolution
and picture quality. This is because that the electron cloud that is formed reacts with the impurities
to create negative ions. These ions have very low mobility compared to electrons (practically
immobile). This result in a decrease in the ionization current that travels toward the anode wires,
which means that information is lost and it is hard to create a good picture. [41]
Xenon can be purified by using the schematic picture shown in figure 7 below.
14
Figure 7. Shows a purification system to achieve sub-ppb purity level of liquid xenon. [18]
At first the liquid xenon is stored and prepared for the purification system in Storage cylinders. The
Xenon is purified in three main steps, first by an oxisorb which removes impurities by
chemisorptions. Then it enters a molecular sieve trap (which operates around -50 °C) for example of
A4 zeolite model, which increases the adsorption capability of zeolites. Finally the xenon enters a ZrV-Fe getter (which is activated at 400 °C) to remove the last part of impurities by another step of
chemisorptions. How high purity level of liquid xenon which is achieved is dependant of how many
cycles of the purification system run. The number of cycles needed to reach sub-ppb levels is
dependant of many things, for example the high level of purity of the input xenon, the saturation of
the zeolites in the molecular sieve, etc. The molecular sieve trap is limited to a few cycles to minimize
the loss of xenon molecules, which also could be adsorbed along with impurities. To achieve the
different temperatures needed in the purification system both cooling system using liquid nitrogen
and high performing isolation around the heated parts using high vacuum isolation (double wall
isolation with 10-6 torr in between) is required. [34]
An issue that must be considered is if it is enough to purify the xenon once before it is inserted in the
PET-scanner or if it must be cleaned regularly. Preferably only one cleaning step is needed, however
this requires that the containers do not contaminate the liquid xenon over time. This requires that
the metal used is extremely clean and also that if a wave shifter in placed on the inside of the
container the film does not fall off and contaminate the xenon.
15
4.1.3. Cost and Availability
When calculating the cost for a liquid xenon TOF-PET apparatus a lot of different applications and
instrument have to be considered. Those are listed below.
 Scintillation material for the apparatus, liquid xenon
 Photomultiplier tubes, PMT together with wave shifter alt. LAAPD
 Container for liquid xenon
 Electrode system
 Cooling system
 Purification system including oxisorb, molecular sieve etc.
 Software and hardware for interpreting data
The annual world production of xenon today is 9000
gas [42] which corresponds to 16
liquid
[43]. The advantage with xenon production is that it can be done wherever around the globe. One
TOF-PET using liquid xenon as scintillator would need a significant volume of xenon to fill all
containers in the apparatus.
Depending on the most convenient solution, either PMTs together with a wave shifter or LAAPD will
be used. Philips has said that it would be best if the emission wavelength is between 400-1000nm,
because they then would be able to use their own silicon based PM. In order to have a wavelength
matching the requirement a wave shifter will be needed to convert the scintillation light from xenon,
which is emitted at 178nm.
The price of xenon varies a lot and depends on many factors, for example the purity level, the
amount of xenon, liquid or gas form, etc.
A bottle of 1 liter xenon gas with a pressure of 10 bar (approximately 10 liters at atmospheric
pressure) with a purity level of 99,998 % xenon costs about 3600 SEK. A bottle of 10 liters with the
same pressure and purity level would cost about 19 300 SEK. [44]
The size of a cylinder shaped PET-scan is a patient port of 70 cm, an estimated length of 20 cm and an
estimated thickness of liquid xenon of 20 cm. [45]
A volume requirement of liquid xenon is therefore:
(
)
m3 in liquid phase = 62.205 m3 (or 347.352 kg) in gas phase (at 1 atm and 15 °C). [46]
A total cost requirement of liquid xenon is then:
Note that 12 MSEK is just the material cost for the liquid xenon. Purification system, cooling system,
container material, etc. is then an additional cost not counted in the 12 MSEK price.
16
4.1.4. Challenges
4.1.4.1.
Cooling/Pressurizing
To get the xenon to liquid phase the temperature of the chamber must be at least below the boiling
point of -108 at 1 atm pressure [47]. Thus it is important to have a system that will keep the
container with xenon at a constant low temperature.
This could be achieved by using liquid nitrogen as cooling media. An alternative would be to
pressurize the xenon to get a higher boiling point. One drawback when increasing the pressure is that
the material used must be very robust and new safety precautions have to be considered. Increasing
the pressure will also decrease the density of the xenon. In order to pressurize the xenon to
approximately room temperature a pressure of 56 bar is needed, which is a significant pressure [26].
The most probable solution of the cooling/pressurizing challenge would be to pressurize the system
so that the xenon would be liquid at about -50 . With this solution a much cheaper cooling media,
such as dry ice, could be used to keep the temperature of the container.
4.1.4.2.
Purity
When using liquid xenon the required purity is very high, sub-ppb levels of O2 equivalents. The
reason for this is the fact that the described system collects the electrons that are excited when
struck by a photon by applying a current in the containers. In order to be able to collect the electron
cloud no contaminants can be in the clouds way. If so the electron cloud will be "trapped" and the
crucial information that the electron cloud gives is then lost.
Therefore a powerful purification system is needed to be able to achieve this high purity. In addition
to this the containment must be very well sealed so that no contaminants can enter to containers. A
description of such a system was presented in the "technical implementation" section earlier in the
report.
4.1.4.3.
Emission wavelength
In order to be able to use Philips own photo detector a wave shifter will be needed to convert the
emission from the liquid xenon at 178nm to approximately 400nm.
A promising wave shifter that shifts light in these wavelength areas are p-terphenyl [38]. It has an
external quantum efficiency of 100% which is good.
17
4.2.
Alternatives to liquid Xenon
When participating in the mid-term review a lot of valuable comments on the presented work were
received. The Philips team thought that the principle with multiple information collection was very
interesting.
The main problem with liquid xenon from Philips perspective was the fact that it is a liquid. When
using a liquid as a detector material some new things have to be considered. For example the
containment of the detector will be a new design that never has been used before. Another issue
that Philips did not like was the need of either cooling or pressurized system.
Therefore a wish from the Philips crew was a material that is solid but still uses the same technique
used with the liquid xenon.
This material should therefore be a transparent solid material with high density and be a scintillator.
Most important is that the material should have the special ability that it both scintillates and that it
is possible to collect drifting electrons to be able to achieve double information. Of course the wish
from Philips is that the material can be produced in rectangular blocks that fit a modified version of
the existing apparatus and that the scintillating light is in the range that is possible to detect with the
Philips detector.
4.2.1. Possible alternatives
Finding a substance that fulfills the requirements to be as good as liquid xenon was a very hard task.
A crystal clear solution has not been found but two possible candidates will be presented.
4.2.1.1.
ITO – Indium Tin Oxide
Todays production of ITO is mainly thin films. They are transparent and are used for different
displays, for example LCD, LED and OLED [48].
The advantages with ITO as a substitute to liquid xenon are the density that is between 7-8
[2]
and the fact that ITO is transparent (at least when produced as thin films).
The challenges with ITO is that it has never been produced in bulk, only thin films have been reported
with a thickness of about 150-300 nm [48]. The scintillating properties of ITO have never been
investigated.
The proposal is therefore to try and produce ITO doped with a scintillating substance (For instance
) in bulk and run experiments to see if the material could be a substitute to liquid xenon.
Interesting factors to look at is the transparency when ITO is produced in bulk and also if there is
enough scintillating light that can be used in the apparatus.
4.2.1.2.
CZT – Cadmium Zink Tellurium
As another alternative to liquid xenon CZT have been investigated. CZT is commonly used as a
photoelectric detector in PET applications. CZT is a semiconductor and is not proved to be a
scintillating material. Due to this the technology used for liquid xenon is not directly applicable for
CZT.
18
Still CZT is an interesting alternative. It has a density of 5.8
. It has been described that it is
possible to get double information when using CZT as a photoelectric detector. Due to the fact that
CZT is a photoelectric detector it is possible to get readings from all Compton scatter events of every
photon. This information together with Com ton kinemati s allows the user to “ba ktra k” the origin
of the primary photon. It is done by combining a conventional PET scan and a so called Compton
camera. The Compton camera is performing the backtracking calculations [49][50].
This system is not the same as the one used for liquid xenon but it takes away the problem with
Compton scatter that is most unwanted in ordinary PET systems. It is not wanted because it is not
possible to handle and will create noise and increase the energy resolution. Due to the possibility of
getting rid of the “compton s atter roblem” the energy resolution will be im roved signifi antly.
This was also one of the nice features in the liquid xenon detection system.
The price of CZT is however very high. If a production process is used that leads to a minimal amount
of defects the price is approximately 2000 $/cm3. [51] If it is assumed that the crystals have the same
dimension as today, and are as many, the total material cost can be calculated as:
This price is based on information from 2003, which means that the actual price may differ from the
one calculated above.
Figure 8. CZT Crystal arrangement. [52]
Figure 8 shows a possible way to arrange the crystals in the PET scanner. The main idea is to keep the
design used today with a lot of smaller crystals. The difference is that CZT requires anode and
cathode strips to gather electrons.
19
5. Conclusion
The task of finding a replacement for LYSO has proven itself to be a very difficult job. In order to be
able to find a substitute the work was very generally at start. Although a lot of different alternatives
were discarded after a short research. Alternatives that were investigated and discarded were for
example semiconductors, heavy glasses, ionic liquids and SrI2. All these material were on par with
LYSO on some criteria but there were always one or more criteria that were not fulfilled.
One material that showed promising performance was liquid xenon. This solution was of course an
“outside of the box” solution. There are a lot of differen es when om aring liquid xenon with a
conventional scintillator. The first issue is the fact that the scintillator is a liquid, and also that it is
only in liquid form at very low temperatures.
The advantage with liquid xenon compared to LYSO is the way of collecting information from the
scintillating event. When using liquid xenon, both the scintillating light and a created ionization
current are collected. This double information eliminates the problem with Compton scatter that is
one big issue when using LYSO. When only considering the detection performance liquid xenon
shows very promising results.
The main challenges with liquid xenon are, as mentioned above, the need of cooling and the
containment of the liquid xenon. Another drawback when using liquid xenon is the need of very high
purity. When the xenon is not pure enough the contaminants will interact with the drifting electrons
and hence reduce the ionization current which is needed to get full information.
In the report a possible technical solution for using liquid xenon has been presented. This includes a
three step purification system that achieves sub-ppb levels of contaminants. The containment is
done by using twelve different containers. Inside the containers there are different wires that collect
the drifting electrons and on the sides of the containers are the photo detectors that detect the
scintillating light.
When presenting the idea with liquid xenon for Philips they liked some parts of the presented
material. The major discovery was the way of collecting double information. What Philips did not like
was the usage of liquid xenon. Therefore their request was to find another solid material that could
be used in the same way as the liquid xenon.
For a future solution two different solutions have been presented.
The first one is to try to dope ITO with a scintillating substance. This could be a promising scintillator,
but experiments are required to confirm this.
The second alternative is not a scintillator but a photoelectric detector called CZT. It has been shown
that it is possible to combine the conventional detector design for CZT with a Compton camera. The
Compton camera uses Compton kinematics to backtrack the origin of the photon. This is not the
same technique as in the liquid xenon system but eliminates the problem with Compton scatter.
However, CZT is very expensive if a high quality is needed and is thus eliminated as an alternative if
not a cheaper production technique is found.
20
6. Reference
[1] Philips healthcare webpage, http://www.philips.se/about/company/companyprofile.page, 201205-11
[2] Philips healthcare webpage, http://www.philips.se/about/company/history/index.page, 2012-0511
[3] Philips healthcare webpage,
http://www.healthcare.philips.com/us_en/products/nuclearmedicine/index.wpd, 2012-05-11
[4] Positron Emission Tomography, Terry Jones, Nuclear Instruments and Methods in Physics
Research A273 (1988) 455-458
[5] Recent Developments in PET Instrumentation, Hao Peng and Craig S. Levin, Current
Pharmaceutical Biotechnology, 2010, 11, 555-571
[6] PETNET solutions, http://www.petscaninfo.com/zportal/portals/pat/my_pet_scan/preparing ,
2012-05-08
[7] Scintillator Challenge Project Milestones, PowerPoint handouts 12-01-2012
[8] Scintillation Crystals for PET, Charles L. Melcher, CTI Inc., Knoxville, Tennessee
[9] The Scintillator Project (Healthcare), PowerPoint, Version: 2011-11-18, Harryson Consulting
Innovation Challenge
[10] Compton Scattering Wikipedia. http://en.wikipedia.org/wiki/Compton_scattering
2012-05-02
[11] Scionix, http://www.scionix.nl/scintillator.htm 2012-05-08
[12] Wide band gap scintillation materials: Progress in the technology and material understanding,
M.Nikl, Institute og physics, Academy of sciences of the Czech Republic, phys. Stat. sol. (a) 178,595
(2000)
[13] Test Procedure for Scintillation Radiation Detectors, Francisco Javier Ramírez Jiménez, Instituto
Nacional de Investigaciones Nucleares,MÉXICO, December 2008
[14] Nationalencyklopedin, http://www.ne.se/lang/scintillation, 2012-05-08
[15] Reine Wallenberg, Professor and Director of solid state chemistry at LTH, 2012
[16] The Scintillator challenge, PowerPoint presentation from Philips
[17] Temperature dependence of the fast, near band edge scintillation from CuI, HgI2 , PbI2, ZnO:Ga
and CdS:In, Stephen E. Derenzo et al, Nuclear Instruments and Methods in Physics Research A 486
(2002) 214-219
[18] Development of ZnO:Ga as an ultra-fast scintillator, E.D. Bourret-Courchesne et al. Nuclear
Instruments and Methods in Physics Research A 601 (2009) 358-363
21
[19] Eu3+-activated heavy scintillating glasses, J. Fu et. al., Materials Research Bulletin 43 (2008)
1502-1508, 2008
[20] Terbium-activated heavy scintillating glasses, . J Fu et. al., Journal of luminescence 128 (2008)
99-104
[21] Luminescent Ionic Liquids, Slawomir Pitula aus Bydgoszcz (Poland), Köln 2009
[22] Fabrication and properties of translucent SrI2 and Eu: SrI2 scintillator ceramics, Stephen R.
Podowitz et al., IEEE transactions on nuclear science, Vol 57, NO. 6, december 2010
[23] Radioactive contamination of SrI2 (Eu) crystal scintillator, P. Belli et al., Nuclear Instruments and
Methods in Physics Research A 670 (2012) 10-17
[24] Liquid Xenon scintillators for imaging of positron emitters, Louis Lavois et.al, The Franklin
McLean Memorial Research Institute, The University of Chicago, Medical Physics, Vol 3, No. 5,
Sept/Oct. 1976
[25] Staffan Hansen, Professor in Inorganic Chemistry at Lund University
[26] Scintillation yield of liquid xenon at room temperature, K. Ueshima et al, arXiv:0803.2888v1
[physics.ins-det] 19 Mar 2008
[27] Liquid xenon detectors for particle physics and astrophysics, E. Aprile and T. Doke, The American
Physical Society, Volume 82, July-September 2010
[28] Compton Positron Emission Tomography with a Liquid Xenon Time projection chamber, K.
Giboni et al. Institute of Physics and SISSA, 2007
[29] A new liquid xenon scintillation detector for PET, Vitaly Yu. Chepel, Nucl. Tracks Radiat. Meas.,
Vol. 21, No.1 , pp. 47-51, 1993
[30] Inorganic Scintillators: today and tomorrow, Marvin J. Weber, Journal of Luminescence, 100
(2002) 35-45
[31] Liquid Xenon detectors for positron emission tomography, A. Miceli et al. Journal of physics:
Conference series 312 (2011)
[32] A liquid Xenon detector for micro-PET, F. Retière et al. Nuclear Instruments and Methods in
Physics Research A 623 (2010) 591-593
[33] Radiation detectors, A collaborative homework for the lectures on measurement methods in
nuclear physics, Bitterling, Garc__a, G•orgen, Holl, Ingenhaag, Lang, L•oher, Omet, Vogelmann.
[34] Detection of gamma-rays with a 3.5 l liquid xenon ionization chamber triggered by the primary
scintillation light, Elena Aprile, Aleksey Bolotnikov, Danli Chen, Reshmi Muhkerjee, Fang Xu, Nuclear
Instruments and methods in Physics Research A 480 (2002) 636-650.
[35] A liquid xenon TPC for a medical imaging Compton telescope, T. Oger et al. Nuclear instruments
and methods in physics research Section A (2012), doi: 1.1016/j.nima.2011.12.004
22
[36] Measurement of attenuation length of scintillation light in liquid xenon, N. Ishida et al, Nuclear
Instruments and Methods in Physics Research A327 (1993) 152-154
[37] Detection of scintillation light of liquid xenon with a LAAPD, V.N. Solovov et al, Nuclear
Instruments and Methods in Physics Research A 488 (2002) 572-578
[38] UV quantum efficiencies of organic fluors, C.H. Lally et al. Nuclear instruments and Methods in
Physics Research B 117 (1996) 421-427
[39] Performance of a large area avalanche photodiode in a liquid xenon ionization and scintillation
chamber, K. Ni et al, Nuclear Instruments and Methods in Physics Research A 551 (2005) 356-363
[40] Kerry, Frank G. (2007). Industrial Gas Handbook: Gas Separation and Purification. CRC Press.
sid. 101–103.
[41] Purification of liquid xenon and impurity monitoring for a PET detector, V.Y. Chepel, M.I. Lopes,
R. Ferreira Marques, A.J.P.L. Policarpo, Nuclear Instruments and Methods in Physics Research A 349
(1994) 500 – 505 North-Holland.
[42] CryoGas International 2007,
http://bgcspecgas.com/media/Rare$20Gases$20Supply$20$26$20Demand$206-07.pdf 2012-03-05
[43] AirLiquide encyclopedia webpage for different gases,
http://encyclopedia.airliquide.com/encyclopedia.asp?LanguageID=11&CountryID=19&Formula=&Ga
sID=71&UNNumber=&EquivGasID=71&VolLiquideBox=&MasseLiquideBox=&VolGasBox=9000&Mass
eGasBox=&btnGasToLiquid=Calculate&RD20=29&RD9=8&RD6=64&RD4=2&RD3=22&RD8=27&RD2=
20&RD18=41&RD7=18&RD13=71&RD16=35&RD12=31&RD19=34&RD24=62&RD25=77&RD26=78&R
D28=81&RD29=82 2012-03-05
[44] Ingela Tesch, AirLiquide, mailcontact 2012-02-08
[45]Philips Healthcare webpage for PET/CT scans,
http://www.healthcare.philips.com/in_en/products/nuclearmedicine/products/geminitfready/index.
wpd#&&/wEXAQUOY3VycmVudFRhYlBhdGgFFkRldGFpbHM6U3BlY2lmaWNhdGlvbnOMsm5HSTlfUhi
ZBla88BAYqeUicw== 2012-05-08
[46] AirLiquide encyclopedia webpage for different gases,
http://encyclopedia.airliquide.com/encyclopedia.asp?LanguageID=11&CountryID=19&Formula=Xe&
GasID=0&UNNumber=&EquivGasID=71&VolLiquideBox=0.452389342&MasseLiquideBox=&btnLiquid
ToGas=Calculate&VolGasBox=&MasseGasBox=&RD20=29&RD9=8&RD6=64&RD4=2&RD3=22&RD8=2
7&RD2=20&RD18=41&RD7=18&RD13=71&RD16=35&RD12=31&RD19=34&RD24=62&RD25=77&RD2
6=78&RD28=81&RD29=82 2012-05-08
[47] Liquid xenon detectors for particle physics and astrophysics, E. Aprile and T. Doke, The American
Physical Society, Volume 82, July-September 2010, Table 1
[48] Electrical and optical properties of indium tin oxide films prepared on plastic substrates by radio
frequency magnetron sputtering, Chih-hao Yang et al., Thin solid films 516 (2008) 1984-1991¨’
23
[49] Simulation studies of CZT detectors as gamma-ray calorimeter, I. Jung et al., Astroparticle
Physics 26 (2006) 119-128
[50] Simulation for CZT Compton PET (Maximization of the efficiency for PET using compton event),
Changyeon Yoon, Wonho Lee and Taewoong Lee, Nuclear Instruments and Methods in Physics
Research A 652 (2011) 713-716
[51] Cadmium Zinc Telluride (CZT) Detectors, Lee Sobotka, Walter Reviol, Oak Ridge 2003,
http://www.pas.rochester.edu/~cline/ria/Reviol.pdf 2012-05-25
[52] Recent developments in PET detectors technology, Tom K Lewellen, Physics in medicine and
biology 53 (2008) R287-R317
24